Semiconductor light emitting device and method of fabricating the same

ABSTRACT

A semiconductor light emitting device of the present invention comprises a n-type InP substrate ( 1 ), and a stripe structure ( 10 ) formed in the stripe shape on the n-type InP substrate ( 1 ) and comprised of a n-type InP lower cladding layer ( 3 ), an active layer ( 4 ) having a resonator in a direction parallel to the n-type InP substrate ( 1 ), and a p-type InP upper cladding layer ( 5 ). The stripe structure ( 10 ) has a photonic crystal structure ( 2 ) with concave portions  9  arranged in rectangular lattice shape, and the direction in which the concave portions ( 9 ) of the photonic crystal structure ( 2 ) are arranged corresponds with a resonator direction. A stripe-shaped upper electrode ( 6 ) is formed on the stripe structure ( 10 ) to extend in the resonator direction. The semiconductor light emitting device of the present invention so structured is configured to radiate light in the direction perpendicular to the n-type InP substrate ( 1 ).

[0001] This is a continuation application under 35 U.S.C.111(a) ofpending prior International Application No. PCT/JP03/01285, filed onFeb.7, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present relates to a semiconductor light emitting device of asurface emitting type using photonic crystal grown on a substrate and amethod of fabricating the same.

[0004] 2. Description of the Related Art

[0005] The conventional semiconductor light emitting devices usingphotonic crystal are disclosed in, for example, Japanese Laid-OpenPatent Application Publication No. Hei. 11-330619, Japanese Laid-OpenPatent Application Publication No. 2001-308457, Japanese Laid-OpenPatent Application Publication No. 2001-9800 (U.S. patent PublicationNo. 2002/0109134 specification), Japanese Laid-Open Patent ApplicationPublication No. Sho 63-205984 (U.S. Pat. No. 4,847,844 specification),and Japanese Laid-Open Patent Application Publication No. 2002-062554.

[0006] Likewise, the semiconductor light emitting device is alsodisclosed in “Imada et al., Applied Physics Letters 75 (1999) 316 (Appl.Phys. Lett. 75 (1999) 316).” FIG. 1 is a perspective view showing astructure of the conventional semiconductor light emitting device usingphotonic crystal disclosed in this “Imada et al.” As shown in FIG. 1, ona n-type InP substrate 51, a n-type InP photonic crystal layer 52, an-type InP lower cladding layer 53, a quantum well active layer 54comprised of InGaAsP, and a p-type InP upper cladding layer 55 aresequentially disposed. On a rear surface of the n-type InP substrate 51,a lower electrode 57 is formed, and on a front surface of a p-type InPupper cladding layer 55, a circular upper electrode 56 having a diameterof approximately 350 μm is formed. Further, on a n-type InP photoniccrystal layer 52, a plurality of circular concave portions 59 eachhaving a diameter of 0.2 μm are periodically formed.

[0007] The n-type InP photonic crystal layer 52 is grown on the n-typeInP substrate 51 to have the plurality of concave portions 59, while thep-type InP upper cladding layer 55, the quantum well active layer 54,and the n-type InP lower cladding layer 53 are grown in this order onanother substrate. Then, the n-type InP lower cladding layer 53 isbrought into surface contact with the n-type InP photonic crystal layer52, and they are annealed in hydrogen atmosphere to be fusion bonded toeach other (see arrow 60). Thereafter, the substrate is removed from thesubstrate and the p-type InP upper cladding layer 55 grown thereon, thecircular upper electrode 56 is formed on a surface thereof, and thelower electrode 57 is formed on the rear surface the n-type InPsubstrate 51, thereby fabricating a semiconductor light emitting devicestructured as described above.

[0008] Upon conducting a current between the upper electrode 56 and thelower electrode 57 in the semiconductor light emitting device fabricatedas described above, at a threshold current of 2A or more, stimulatedemission is observed and a single mode with an oscillation wavelength of1.3 μm is gained. Light emits from an outer peripheral portion 58 of theupper electrode 56.

[0009] As should be understood, in the conventional semiconductor lightemitting device, the threshold current is relatively large, for example,2A.

[0010] In addition, since the upper electrode 56 is circular,polarization planes of light have different directions. The polarizationplanes may be oriented in the same direction by forming the concaveportions 59 in the shape of oval, but the plurality of concave portions59 which have the same oval shape are very difficult to create.

[0011] Even in the light having the same polarization plane, the lightemitting wavelength becomes unstable because of the presence of twostable light emitting modes.

[0012] Although the semiconductor light emitting device is fabricated byfusion bonding crystals to each other as described above, the entiresurfaces of the substrates having a large diameter are difficult tofusion bond.

SUMMARY OF THE INVENTION

[0013] The present invention has been developed under the circumstances,and an object of the present invention is to provide a semiconductorlight emitting device with a low threshold current and with polarizationplanes of light controlled, and a method of fabricating thesemiconductor light emitting device.

[0014] In order to achieve the above described object, there is provideda semiconductor light emitting device comprising a semiconductorsubstrate; a semiconductor layered structure provided on thesemiconductor substrate and comprised of a lower cladding layer, anactive layer having a resonator in a direction parallel to thesemiconductor substrate, and an upper cladding layer; an upper electrodeconnected to the upper cladding layer and extending in stripe shape in aresonator direction; and a lower electrode connected to the lowercladding layer, wherein the semiconductor layered structure has aphotonic crystal structure in which a plurality of concave portions orconvex portions are arranged periodically in the resonator direction,the photonic crystal structure is configured such that at least part ofthe photonic crystal structure does not overlap with the upper electrodeand the photonic crystal structure and the upper electrode are arrangedin the resonator direction as seen in a plan view, and when apredetermined voltage is applied between the upper electrode and thelower electrode, light radiates from a region of the photonic crystalstructure which does not overlap with the upper electrode as seen in aplan view.

[0015] It is preferable that in the semiconductor light emitting device,the concave portions or the convex portions are formed in the uppercladding layer.

[0016] It is preferable in that in the semiconductor light emittingdevice, the concave portions or the convex portions are formed in theupper cladding layer, the active layer, and the lower cladding layer.

[0017] It is preferable that in the semiconductor light emitting device,the concave portions or the convex portions are cylindrical.

[0018] It is preferable that in the semiconductor light emitting device,the concave portions or the convex portions are flat-plate shaped.

[0019] It is preferable that in the semiconductor light emitting device,the resonator has a width of not less than 2 μm and not more than 10 μm.

[0020] It is preferable that in the semiconductor light emitting device,wherein the resonator has a length of not less than 20 μm and not morethan 50 μm.

[0021] It is preferable that in the semiconductor light emitting device,the resonator direction is <110> direction or <−110> direction.

[0022] It is preferable that in the semiconductor light emitting device,the concave portions or convex portions are arranged in the shape ofrectangular lattice such that one arrangement direction of the concaveportions or the convex portions corresponds with the resonator directionand another arrangement direction is perpendicular to the resonatordirection.

[0023] It is preferable that in the semiconductor light emitting device,the spacing between adjacent concave portions or convex portions in theone arrangement direction is different from the spacing between adjacentconcave portions or convex portions in the another arrangementdirection.

[0024] It is preferable that in the semiconductor light emitting device,the spacing between adjacent concave portions or convex portions in theone arrangement direction is larger than the spacing between adjacentconcave portions or convex portions in the another arrangementdirection.

[0025] It is preferable that in the semiconductor light emitting device,reflection films are provided on both end faces of the semiconductorlayered structure.

[0026] It is preferable that in the semiconductor light emitting device,the semiconductor layered structure is provided with a photonic crystalstructure on a periphery thereof, and the photonic crystal structure iscomprised of a plurality of concave portions or convex portions arrangedat a predetermined spacing.

[0027] It is preferable that in the semiconductor light emitting device,the concave portions or the convex portions are provided over an entireupper surface of the semiconductor layered structure.

[0028] It is preferable that in the semiconductor light emitting device,the region of the photonic crystal structure that does not overlap withthe upper electrode as seen in a plan view is located at a centerportion of the semiconductor layered structure.

[0029] It is preferable that in the semiconductor light emitting device,a spacing between part of the concave portions or convex portionsadjacent in the resonator direction is larger than a spacing betweenanother concave portions or convex portions by a wavelength/(actualrefractive index×4).

[0030] It is preferable that the semiconductor light emitting devicecomprises a plurality of semiconductor layered structures arranged tocross one another.

[0031] According to the present invention, there is provided a method offabricating a semiconductor light emitting device comprising: asemiconductor substrate; a semiconductor layered structure provided onthe semiconductor substrate and comprised of a lower cladding layer, anactive layer having a resonator in a direction parallel to thesemiconductor substrate, and an upper cladding layer; an upper electrodeconnected to the upper cladding layer; and a lower electrode connectedto the lower cladding layer, wherein light radiates in a directionsubstantially perpendicular to the semiconductor substrate, the methodcomprising the steps of epitaxially growing the semiconductor layeredstructure on the semiconductor substrate; etching the semiconductorlayered structure to form a photonic crystal structure comprised of aplurality of concave portions arranged periodically in a resonatordirection; and forming the upper electrode on the upper cladding layerso as to extend in stripe shape in the resonator direction such that theupper electrode does not overlap with at least part of the photoniccrystal structure and the upper electrode and the photonic crystalstructure are arranged in the resonator direction as seen in a planview.

[0032] According to the present invention, there is further provided amethod of fabricating a semiconductor light emitting device comprising:a semiconductor substrate; a semiconductor layered structure provided onthe semiconductor substrate and comprised of a lower cladding layer, anactive layer having a resonator in a direction parallel to thesemiconductor substrate, and an upper cladding layer; an upper electrodeconnected to the upper cladding layer; and a lower electrode connectedto the lower cladding layer, wherein light radiates in a directionsubstantially perpendicular to the semiconductor substrate, the methodcomprising the steps of epitaxially growing the semiconductor layeredstructure on the semiconductor substrate; selectively growing crystal onthe upper cladding layer of the semiconductor layered structure to forma photonic crystal structure comprised of a plurality of concaveportions arranged periodically in the resonator direction; and formingthe upper electrode on the upper cladding layer so as to extend instripe shape in the resonator direction such that the stripe-shapedupper electrode does not overlap with at least part of the photoniccrystal structure and the upper electrode and the photonic crystalstructure are arranged in the resonator direction as seen in a planview.

[0033] The above and further objects and features of the invention willbe more fully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a perspective view showing a structure of theconventional semiconductor light emitting device using the conventionalphotonic crystal;

[0035]FIG. 2 is a plan view showing a structure of a semiconductor lightemitting device according to a first embodiment of the presentinvention;

[0036]FIG. 3 is a view taken in the direction of arrows along lineIII-III in FIG. 2;

[0037]FIG. 4 is a view taken in the direction of arrows along like IV-IVin FIG. 2;

[0038]FIGS. 5A to 5D are views for explaining a method of fabricatingthe semiconductor light emitting device according to the firstembodiment of the present invention, FIG. 5A is a cross-sectional viewshowing a structure of the semiconductor light emitting device and FIGS.5B to 5D are plan views showing a structure of the semiconductor lightemitting device;

[0039]FIG. 6 is a cross-sectional view showing a structure of amodification of the semiconductor light emitting device according to thefirst embodiment of the present invention;

[0040]FIGS. 7A and 7B are views for explaining a light emitting state ofthe semiconductor light emitting device according to the firstembodiment of the present invention, in which FIG. 7A is a view showingthe relationship between wave number and energy of light in a regionwithout photonic crystal and FIG. 7B is a view showing the relationshipbetween wave number and energy of light in a photonic crystal structure;

[0041]FIG. 8 is a plan view showing a structure of a semiconductor lightemitting device according to a second embodiment of the presentinvention;

[0042]FIGS. 9A and 9B are views for explaining a method of fabricatingthe semiconductor light emitting device according to the secondembodiment of the present invention, FIG. 9A is a cross-sectional viewshowing a structure of the semiconductor light emitting device and FIGS.9B to 9D are plan views showing a structure of the semiconductor lightemitting device;

[0043]FIGS. 10A and 10B are views for explaining a light emitting stateaccording to the second embodiment of the present invention, in whichFIG. 10A is a view showing the relationship between wave number andenergy of light in a photonic crystal structure in a resonatordirection, and FIG. 10B is a view showing the relationship between wavenumber and energy of light in the photonic crystal structure in adirection perpendicular to the resonator direction;

[0044]FIGS. 11A and 11B are views showing a structure of a semiconductorlight emitting device according to a third embodiment of the presentinvention, in which FIG. 11A is a plan view and FIG. 11B is a view takenin the direction of arrows along line A-A in FIG. 1IA;

[0045]FIGS. 12A and 12B are views showing a structure of modification ofthe semiconductor light emitting device according to the thirdembodiment of the present invention, in which FIG. 12A is a plan viewshowing a structure thereof and FIG. 12B is a view taken in thedirection of arrows along line B-B in FIG. 12A;

[0046]FIGS. 13A and 13B are views showing a structure of a semiconductorlight emitting device according to a fourth embodiment of the presentinvention, in which FIG. 13A is a plan view and FIG. 13B is a view takenin the direction of arrows along line C-C in FIG. 13A;

[0047]FIGS. 14A to 14E are views showing a modification of asemiconductor light emitting device according to a fourth embodiment ofthe present invention, in which FIGS. 14A to 14D are plan views showinga structure thereof and FIG. 14E is a view showing change in lightintensity in an internal portion of a stripe structure of thesemiconductor light emitting device;

[0048]FIGS. 15A to 15C are views showing a structure of a semiconductorlight emitting device according to a fifth embodiment of the presentinvention, in which FIG. 15A is a plan view showing a structure thereof,FIG. 15B is a cross-sectional view showing a structure of modification,and FIG. 15C is a view showing change in light intensity in an internalportion of a stripe structure of the semiconductor light emittingdevice;

[0049]FIGS. 16A and 16B are views for explaining a light emitting stateof the semiconductor light emitting device according to the fifthembodiment of the present invention, FIG. 16A is a view showing therelationship between wave number and energy of light in the photoniccrystal structure in a α direction (resonator direction), and 16B is aview showing the relationship between wave number and energy of light inthe photonic crystal structure in a β direction (direction perpendicularto the resonator direction);

[0050]FIGS. 17A to 17C are views for explaining a semiconductor lightemitting device according to a sixth embodiment of the presentinvention, in which FIG. 17A is a plan view showing a structure of thesemiconductor light emitting device, FIG. 17B is a view showing anoperation principle of the semiconductor light emitting device, and FIG.17C is a view showing behavior of an electric field;

[0051]FIGS. 18A and 18B are views showing a structure of a modificationof the semiconductor light emitting device according to the sixthembodiment of the present invention, in which FIG. 18A is a plan viewshowing a structure of the modification and FIG. 18B is a plan viewshowing a structure of another modification;

[0052]FIGS. 19A to 19D are views for explaining a method of fabricatinga semiconductor light emitting device according to a sixth embodiment ofthe present invention, in which FIG. 19A is a cross-sectional viewshowing a structure of the semiconductor light emitting device and FIGS.19B to 19D are plan views showing a structure of the semiconductor lightemitting device;

[0053]FIGS. 20A to 20D are plan views showing a structure of anothermodification of the semiconductor light emitting device according to thesixth embodiment of the present invention; and

[0054]FIGS. 21A to 21D are views for explaining a method of fabricatinga modification of the semiconductor light emitting device according tothe sixth embodiment of the present invention, in which FIG. 21A is across-sectional view showing a structure of the modification, and FIGS.21B to 21D are plan views showing a structure of the modification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] Hereinafter, embodiments of the present invention will bedescribed with reference to the accompanying drawings.

[0056]FIG. 2 is a plan view showing a structure of a semiconductor lightemitting device according to a first embodiment of the presentinvention. FIG. 3 is a view taken in the direction of arrows along lineIII-III in FIG. 2. FIG. 4 is a view taken in the direction of arrowsalong line IV-IV in FIG. 2. In this embodiment, a light emittingwavelength of the semiconductor light emitting device is set to 1.3 μm.

[0057] As shown in FIGS. 2 to 4, a stripe structure 10 is provided on an-type InP substrate 1. The stripe structure 10 has a n-type InP lowercladding layer 3 (100 nm thick), an InGaAsP/InGaAsP quantum well activelayer 4 (hereinafter simply referred to as an active layer 4), and ap-type InP upper cladding layer 5 (50 nm thick), which are disposed inthis order. The active layer 4 has a strained quantum well structurecomprised of five pairs of In_(0.9)Ga_(0.1)As_(0.2)P_(0.8) barrierlayers (thickness: 10 nm, composition wavelength: 1.1 μm, latticestrain: 0%), In_(0.9)Ga_(0.1)As _(0.5)P_(0.5) well layers (thickness: 4nm, quantum well wavelength: 1.3 μm, lattice strain: 1%), and a waveguide (guide) layer. The stripe structure 10 has a cleaved end face, andas a result, the active layer 4 functions as a resonator in thedirection parallel to the n-type InP substrate 1. The p-type InP uppercladding layer 5 is provided with a photonic crystal structure 2obtained by arranging a plurality of cylindrical concave portions 9 inthe shape of rectangular lattice. And, one of arrangement directions (αdirection in FIG. 2) of two arrangement directions (α direction and βdirection in FIG. 2) of adjacent concave portions 9 corresponds with theresonator direction.

[0058] As defined herein, the “resonator direction” refers to adirection expressed as the α direction in FIG. 2, i.e., longitudinaldirection of a rectangular resonator.

[0059] A lower electrode 7 is provided on a rear surface of the n-typeInP substrate 1. Meanwhile, a stripe-shaped upper electrode 6 isprovided on a region of a surface of the p-type InP upper cladding layer5 where the photonic crystal structure 2 is not provided as seen in aplan view. The stripe-shaped upper electrode 6 has a width smaller thanthat of the p-type InP upper cladding layer 5. The photonic crystalstructure 2 and the upper electrode 6 are arranged in the resonatordirection as seen in the plan view.

[0060] It should be noted that, when the active layer 4, i.e., theresonator has a width approximately as small as 2 μm, externaldifferential quantum efficiency increases, but intensity of output lightbecomes saturated with an increase in an injected current. On the otherhand, when the width W of the stripe structure 10 is increased toapproximately 10 μm, light can be confined in the lateral direction(width direction of the resonator) within the photonic crystal structure2, output saturation can be inhibited, but light mode in the regionother than the photonic crystal structure 2 becomes unstable. Therefore,when a coupling factor of the photonic crystal structure 2 is relativelysmall, there is a limitation to an increase in the width W of theresonator. Under the circumstance, it is preferable that the width W ofthe resonator is in the range of approximately 2 μm to 10 μm.

[0061] Subsequently, a method of fabricating a semiconductor lightemitting device of this embodiment configured as described above, willbe described.

[0062]FIGS. 5A to 5D are views for explaining a method of fabricatingthe semiconductor light emitting device according to the firstembodiment of the present invention, FIG. 5A is a cross-sectional viewshowing a structure of the semiconductor light emitting device, andFIGS. 5B to 5D are plan views showing a structure of the semiconductorlight emitting device.

[0063] As shown in FIG. 5A, on the n-type InP substrate 1, the Si-dopedn-type InP lower cladding layer 3 (100 nm thick), the undoped activelayer 4 (14 nm thick), and the Zn-doped p-type InP upper cladding layer5 (50 nm thick) are epitaxially grown by a known crystal growth methodsuch as a MOVPE (metalorganic vapor phase epitaxy) process. The activelayer 4 is intentionally undoped without addition of impurities, for thepurpose of inhibiting absorption of valence band and absorption of freeelectrons.

[0064] Using a SiO₂film as an etching mask, ICP dry etching iscircularly performed using a Cl₂ gas and a CH₄ gas, thereby forming aplurality of cylindrical concave portions 9. In this case, as shown inFIG. 5B, the concave portions 9 are arranged in the shape of rectangularlattice. The region with the plurality of concave portions 9 arrangedperiodically becomes the photonic crystal structure 2. The period of theconcave portions 9 (spacing between adjacent concave portions) isapproximately equal to the wavelength of light. Therefore, in thisembodiment, the period is approximately 1.3 μm.

[0065] As shown in FIG. 5C, in order to form a stripe structure, etchingis performed until it reaches a part of the n-type InP substrate 1 fromthe p-type InP upper cladding layer 5 using an oxalic acid basedetchant. Then, as shown in FIG. 5D, a stripe-shaped Cr/Pt/Au electrodeas the upper electrode 6 is vapor deposited on the front surface of thep-type InP upper cladding layer 5 and the Au—Sn electrode as the lowerelectrode (not shown) is vapor deposited on the rear surface of then-type InP substrate 1. In this case, the upper electrode 6 is formed onthe front surface of the p-type InP upper cladding layer 5 by lift-offprocess so that the upper electrode 6 and the photonic crystal structure2 are arranged in the resonator direction as seen in a plan view.

[0066] As described above, the plurality of concave portions 9 arearranged in the shape of rectangular lattice. As shown in FIG. 5D, the adirection of the arrangement directions of the concave portions 9corresponds with the resonator direction.

[0067] Through the above steps, a semiconductor light emitting device ofthis embodiment can be fabricated.

[0068] When the concave portions 9 are formed as described above and thelength of the photonic crystal structure 2 (α direction) is 2 μm ormore, it is desirable that etching stop in the p-type InP upper claddinglayer 5 so as to not etch the active layer 4, as shown in FIG. 3. Bydoing so, since damage to the active layer 4 can be inhibited, lightemitting efficiency can be improved.

[0069] When the length of the photonic crystal structure 2 (in the αdirection) is 10 μm or less, etching is preferably performed until itreaches the n-type InP lower cladding layer 3 such that the active layer4 is etched through. When the length of the photonic crystal structure 2is thus relatively small, for example, 10 μm or less, the effectproduced by the photonic crystal is reduced. Accordingly, in order tofacilitate coupling of light with photonic crystal, etching is performeduntil it reaches the n-type InP lower cladding layer 3 so that largepart of light distribution is subjected to diffraction due to thephotonic crystal. In this case, damage is caused by dry etching, butafter the etching using the Cl₂ gas, the damaged region is etched usingthe CH₄ gas or the SF₆ gas. Thus, an increase in a threshold current isinhibited.

[0070] In this embodiment, the resonator direction is <110> or <−110>direction with high degree of cleavage. But, when the resonator isformed by dry etching, the resonator direction is not limited.Nonetheless, it is preferable that the resonator direction is <110> or<−110> direction in order to improve verticality of an etching end face.As used herein, the <−110> direction is <⁻ 110> direction. InSpecification and Claims, these mean the same.

[0071] The length of the resonator is set to approximately 20 μm to 50μm. This is due to the fact that, since intensity of light diffractedwithin the photonic crystal structure 2 and taken out is small, lightloss can be significantly reduced by reducing a light loss in the endface of the stripe structure 10 with coating on the end face as comparedto a normal laser, and thereby sufficient light is emitted to outsideregardless of reduced gain in a short resonator. It has been revealedthat when the length of the resonator is 50 μm, the threshold current isapproximately 20 μA, and when the length is 20 μm, there is no currentvalue corresponding to the threshold, and light output is directlyproportional to the injected current.

[0072] While in this embodiment, the cylindrical concave portions areperiodically formed as the photonic crystal, cylindrical convex portionsmay be formed. The use of the cylindrical concave portions rather thanthe cylindrical convex portions can inhibit damage caused by etchingbecause of a smaller volume of etching. In addition, the use of theconvex portions presents a problem associated with intensity. In spiteof this, the use of the cylindrical convex portions advantageouslyincreases a period in arrangement in contrast with the concave portions,because an equivalent refractive index is low. Therefore, when thewavelength of light is as small as 0.85 μm, the convex portions arepreferably used. In these respects, this embodiment is identical toanother embodiment mentioned later.

[0073] The photonic crystal may be formed by periodically formingconcave portions or convex portions that are tubular with rectangularcross-section, instead of forming the cylindrical concave or convexportions. But, as compared to the cylindrical concave or convexportions, the concave or convex portions that are tubular withrectangular cross-section, and have the same shape, are difficult tocreate. The same applies to cylindrical concave or convex portions withan oval cross-section. Variation in the shape increases with increasingellipticity. In this embodiment, therefore, cylindrical shape withperfectly circular cross-section is adopted. In these respects, thisembodiment is identical to another embodiments mentioned later.

[0074] An operation of the semiconductor light emitting devicefabricated as described above will be described. Between the upperelectrode 6 and the lower electrode 7, a voltage is applied under thecondition in which the upper electrode 6 is at a positive potential andthe lower electrode 7 is at a negative potential, thereby causing acurrent to flow through the active layer 4. As a result, electrons areinjected from a n-type region to a p-type region in the active layer 4and holes are injected from the p-type region to the n-type region. Theinjected electrons and holes causes stimulated emission in the vicinityof a p-n joint formed at an interface between the n-type region and thep-type region of the active layer 4. Thereby, light is generated in theactive layer 4. This light is amplified within the active layer 4 anddiffracted within the photonic crystal structure 2 in the directionperpendicular to the n-type InP substrate 1. As a result, light 8 isemitted from the photonic crystal structure 2 toward the directionperpendicular to the n-type InP substrate 1.

[0075]FIGS. 7A and 7B are views for explaining a light emitting state ofthe semiconductor light emitting device according to the firstembodiment, in which FIG. 7A is a view showing the relationship betweenwave number and energy of light in the region without photonic crystaland FIG. 7B is a view showing the relationship between wave number andenergy of light in the photonic crystal structure.

[0076] As shown in FIG. 7A, in the region where the photonic crystal isnot formed, an energy linearly increases with increasing wave number.FIG. 7A shows the relationship between the energy and the wave number ina region with low light energy and the relationship after the light isturned back by the periodic structure. The energy increases withdecreasing wave number. Thereafter, the light is turned back again andthe energy increases with increasing wave number. Since perturbation dueto the photonic crystal does not occur, the wave number and the energycontinuously change and photonic band gap is not formed. When the lightintensity is an abscissa axis and the wavelength is an ordinate axis inFIG. 7A, spontaneous emission light shows a Laurents distribution.

[0077] On the other hand, as shown in FIG. 7B, in the photonic crystalstructure, since perturbation due to the photonic crystal occurs,photonic bandgap (PBG) is formed. Within the photonic bandgap, thespontaneous emission light cannot exist. On the other hand, thespontaneous emission light exists in a range of not less than ahigh-energy end of the photonic bandgap and not more than a low-energyend. An angular velocity coo of light corresponding to a center energyof the photonic bandgap is represented by:

n _(eff)·ω₀ /C=π/Λ when the period of the photonic crystal (spacingbetween adjacent concave portions 9) is Λ,

[0078] where n_(eff) is equivalent refractive index of light and c is avelocity of light.

[0079] The magnitude Δω of the photonic bandgap is represented by:

Δω=2 kc/n _(eff)

[0080] where k is a coupling factor. When the high-energy end of thephotonic bandgap conforms to the energy of the spontaneous emissionlight, a relationship associated with perturbation of the photoniccrystal is added to the relationship between electrons that generate thespontaneous emission light and light, so that transition probability ofthe electrons and light are subjected to perturbation. As a result, asshown in FIG. 7B, the spontaneous emission light and the photonicbandgap are coupled, thereby obtaining a high intensity of thespontaneous emission light due to super radiation. A half width of thespontaneous emission light is 0.2 μm or less, and intensity of the lightis approximately 30 times or higher.

[0081] While the high-energy end of the photonic bandgap conforms to thewavelength of the spontaneous emission light in FIG. 7B, the sameresults are obtained when the low-energy end of the photonic bandgapconforms to the wavelength of the spontaneous emission light.Nonetheless, in the case where a plurality of concave portions formingthe photonic crystal are arranged in the rectangular lattice shape, itis advantageous that the high-energy end of the photonic bandgapconforms to the wavelength of the spontaneous emission light, sincethere is no influence of degeneration because of large band spacing.

[0082] In the case of the semiconductor light emitting device of thisembodiment, there is no current value as a threshold, and output lightincreases according to an injected current. It has been revealed thatthe external differential quantization efficiency is approximately 60%when the wavelength is 1.3 μm. It has been found that an operationalspeed is 10 GHz when the injected current is 2 mA or more. Sincecoherent light having the same frequency and phase is obtained from alight emitting region, a spot can be restricted to a value correspondingto NA of a lens. Also, polarization planes of TE light are directed inthe direction of the stripe structure (resonator direction). It has beenfound that when a relatively strong current is injected in the form ofpulse, super radiation in which strong light is emitted for picosecondoccurs.

[0083] While in this embodiment, the semiconductor substrate iscomprised of InP crystal, GaAs, GaN or GaP crystal may be used ifprecision in forming the photonic crystal is increased. In addition, thesemiconductor substrate may be of p-type instead of the n-type.Nonetheless, since the n-type crystal generally has a low resistancevalue, it is preferable that the crystal on the side where the photoniccrystal is present between the crystal and the active layer, may be ofn-type, because a current is uniformly injected into the active layer.

[0084] As described above, it is preferable that the length of thephotonic crystal structure (a direction) in the structure shown in FIG.3 (concave portions 9 are formed only within the p-type InP uppercladding layer 5) is 2 μm or more, and the length of the photoniccrystal structure in FIG. 6 (concave portions 9 are formed to extendfrom the p-type InP upper cladding layer 5 to a part of the n-type InPlower cladding layer 3) is 10 μm or less. Therefore, the photoniccrystal structure having the length (a direction) of not less than 2 μmand not more than 10 μm may adopt the structure in FIG. 3 or FIG. 6.Which of these is desirable depends on severity of damage caused by dryetching and degree of flatness of the shape of an etching surface. Forinstance, the structure in FIG. 6 can be obtained by forming a surfacewith high degree of flatness using a Cl₂ gas in dry etching and byremoving a damage region using a CF₄ gas.

Embodiment 2

[0085] In a second embodiment, a description will be given of asemiconductor light emitting device configured to have photonic crystalas in the first embodiment and have a period of the photonic crystal inthe resonator direction and a period of the photonic crystal in thedirection perpendicular to the resonator direction which differ fromeach other.

[0086]FIG. 8 is a plan view showing a structure of a semiconductor lightemitting device according to the second embodiment of the presentinvention. As shown in FIG. 8, the photonic crystal structure 2 isformed by arranging a plurality of concave portions 9 in the shape ofrectangular lattice. Here, a period of the concave portions 9 in theresonator direction (spacing between adjacent concave portions 9) F1 islonger than a period F2 of the concave portions 9 in the directionperpendicular to the resonator direction. In FIG. 8, reference numeral13 denotes a growth region where the p-type InP upper cladding layer isselectively grown as described later. Since the other structure of thesemiconductor light emitting device is identical to that of the firstembodiment, they are identified by the same reference numerals and willnot be described.

[0087] Subsequently, a fabrication method of the semiconductor lightemitting device of this embodiment so structured, will be described. Inthe first embodiment, the fabrication method of the photonic crystalusing the dry etching has been described, while in the secondembodiment, the fabrication method of the photonic crystal usingselective growth will be described. As a matter of course, as in thefirst embodiment, the photonic crystal can be formed by the method usingdry etching. In all embodiments in the specification, the photoniccrystal can be formed by a method using either the dry etching or amethod using selective growth.

[0088]FIGS. 9A to 9D are views for explaining a method of fabricatingthe semiconductor light emitting device according to the secondembodiment of the present invention, FIG. 9A is a cross-sectional viewshowing a structure of the semiconductor light emitting device and FIGS.9B to 9D are plan views showing a structure of the semiconductor lightemitting device.

[0089] As shown in FIG. 9A, on the n-type InP substrate 1, the Si-dopedn-type InP lower cladding layer 3 (100 nm thick), the undoped activelayer 4 (14 nm thick), and the Zn-doped p-type InP upper cladding layer5 (10 nm thick) are epitaxially grown by a known crystal growth methodsuch as the MOVPE process. A SiO₂ film 12 is formed on the p-type InPupper cladding layer 5.

[0090] Subsequently, as shown in FIG. 19B, using the SiO₂ film 12 as aselective growth mask, the p-type InP upper cladding layer 5 (100 nmthick) is selectively grown, thereby forming a plurality of cylindricalconcave portions 9 arranged in the shape of rectangular lattice. Thus,the photonic crystal structure 2 is formed. At this time, the concaveportions 9 are arranged so that the period F1 of the concave portions 9in the direction coincident with the resonator direction is longer thanthe period 2F of the concave portions 9 in the direction perpendicularto the longitudinal direction thereof.

[0091] It should be appreciated that facets might be formed duringselective growth when forming the photonic crystal structure 2 asdescribed above. But, the facets are not formed when the film thicknessis as small as approximately 200 μm or less.

[0092] Subsequently, as shown in FIG. 9C, in order to form a stripestructure, etching is performed until it reaches the part of the n-typeInP substrate 1 using the oxalic acid based etchant. In FIG. 9, asdescribed above, 13 denotes the growth region in which the p-type InPupper cladding layer is selectively grown. Then, as shown in FIG. 9D,the Cr/Pt/Au electrode as the upper electrode 6 is vapor deposited onthe front surface of the p-type InP upper cladding layer 13 in thestripe structure 10. Also, the Au—Sn electrode as the lower electrode 7(not shown) is vapor-deposited on the rear surface of the n-type InPsubstrate 1. In this case, the upper electrode 6 is formed on the frontsurface of the p-type InP upper cladding layer 5 by the lift-off processso that the upper electrode 6 and the photonic crystal structure 2 arearranged in the resonator direction as seen in a plan view.

[0093] Through the above steps, the semiconductor light emitting deviceof this embodiment will be described.

[0094] To obtain the structure with the concave portions 9 extending tothe part of the n-type InP lower cladding layer 3 as shown in FIG. 6,the n-type InP lower cladding layer 3 is first epitaxially grown to havesome thickness, and then using the SiO₂ film 12 as the selective growthmask, the n-type InP lower cladding layer 3, the active layer 4, and thep-type InP upper cladding layer 5 are selectively grown. In this case,in the vicinity of the selective growth mask, the film thicknesses ofthe layers become thin because of the presence of facets, therebycausing a reduced pressure resistance and thereby causing a leakcurrent. Accordingly, preferably, a step of etching and removing theregion having a small thickness is performed. Specifically, after thestep of selective growth, the region having the small thickness isetched using the CH₄ gas or the SF₆ gas. Thereby, an increase in thethreshold current can be inhibited.

[0095] While in this embodiment, the cylindrical concave portions 9 areprovided, cylindrical convex portions may be provided. In the case ofconvex portions, cylindrical crystals are selectively grownindividually. In this case, since the convex portions are individuallygrown, which tends to make the convex portions have different heights.This is problematic in terms of stability in selective growth. Inaddition, since the facets are formed in tip end portions of the convexportions under the condition in which the growth rate is relativelyhigh, flatness is not obtained. In view of these, by carefully selectingthe selective growth conditions, for example, by optimizing the growthcondition such as reducing the growth temperature, the photonic crystalstructure having the cylindrical convex portions should be created.

[0096] In the semiconductor light emitting device fabricated asdescribed above, as in the first embodiment, upon application of thevoltage between the upper electrode 6 and the lower electrode 7,stimulated emission occurs in the active layer 4. Thereby, light isgenerated in the active layer 4. This light is amplified within theactive layer 4 and diffracted within the photonic crystal structure 2 inthe direction perpendicular to the n-type InP substrate 1. As a result,light is emitted from the photonic crystal structure 2 toward thedirection perpendicular to the n-type InP substrate 1.

[0097]FIGS. 10A and 10B are views for explaining a light emitting stateof the semiconductor light emitting device according to the secondembodiment of the present invention, in which FIG. 10A is a view showingthe relationship between wave number and energy of light in the photoniccrystal structure in the resonator direction, and FIG. 10B is a viewshowing the relationship between wave number and energy of light in thephotonic crystal structure in the direction perpendicular to theresonator direction.

[0098] In this embodiment, in the resonator direction, the period Λ ofthe photonic crystal is set such that the wavelength of the spontaneousemission light conforms to the high-energy end of the photonic bandgap.In the direction perpendicular to the resonator direction, the period Λof the photonic crystal is set smaller than that in the directioncoincident with the resonator direction. From n_(eff)·ω/C=π/Λ, thecentral energy (equivalent to ω) of the photonic bandgap increases asshown in FIGS. 10A and 10B.

[0099] As shown in FIGS. 10A and 10B, in the resonator direction, thespontaneous emission light is coupled to the band end, while in thedirection perpendicular to the resonator direction, the wavelength ofthe spontaneous emission light is located within the photonic bandgap.Therefore, in the direction perpendicular to the resonator direction,the spontaneous emission light does not propagate. As a result, sincelight leakage from the stripe structure to outside is reduced,oscillation with a lower threshold current is achieved in contrast withthe case where the period in the resonator direction is equal to theperiod in the direction perpendicular to the resonator direction.

[0100] In the semiconductor light emitting device of this embodiment,with the use of the cylindrical concave portions with perfectly circularcross-section, the polarization planes can be controlled. The firstembodiment illustrates that the TE mode is formed in the resonatordirection, but with an increase in the width of the stripe structure,stability of the mode is degraded. On the other hand, in thisembodiment, by forming the photonic badngap in the directionperpendicular to the resonator direction by varying the pitch of thephotonic crystal, the TE mode does not exist in the directionperpendicular to the resonator direction. This follows that, with thestripe structure having a large width, the TE mode can be induced stablyin the resonator direction.

[0101] In the selective growth, a rectangular structure can be createdwith good repeatability because the facets are formed during growth.Especially when the convex portions are selectively grown, it ispossible to create convex portions having uniform shape by conductingfacet growth. Therefore, when the convex portions are created byselective growth under optimized growth conditions, the photonic crystalstructure comprised of convex portions that are tubular with rectangularcross-section are easily obtained.

Embodiment 3

[0102] In a third embodiment, there is illustrated a semiconductor lightemitting device capable of avoiding leakage of spontaneous emissionlight and stimulated emission light in the resonator direction byforming reflection films on end faces of a stripe structure to reflectlight.

[0103]FIGS. 11A and 11B are views showing a structure of thesemiconductor light emitting device according to the third embodiment ofthe present invention, in which FIG. 11A is a plan view and FIG. 11B isa view taken in the direction of arrows along line A-A in FIG. 11A. Asshown in FIGS. 11A and 11B, insulator multi-layered thin films 11comprising alumina and titania are provided on both end faces of thestripe structure 10. Since the other structure of the semiconductorlight emitting device of this embodiment is identical to that of thefirst embodiment, the same or corresponding parts are identified by thesame reference numerals and will not be further described.

[0104] Subsequently, a method of fabricating the semiconductor lightemitting device of this embodiment structured as described above will bedescribed.

[0105] First of all, the photonic crystal structure 2 is created as inthe first embodiment. Subsequently, the both end faces of the stripestructure are formed to be vertical by dry etching. Then, as shown inFIGS. 11A and 11B, the insulator multi-layered thin films 11 comprisingalumina and titania are formed on the both end faces. Thereafter, thedevice is separated along the dry-etched grooves. In this embodiment, inorder to deposit the insulator multi-layered thin films 11 on thevertical end faces of the stripe structure 10, high reflectionmulti-layered films are formed by an ECR sputtering process. When thenumber of layers forming the multi-layered thin film 11 is four, arefractive index of 98% is gained. By forming such a highly-reflectiveinsulator multi-layered thin films 11, reflection loss at the end facesof the stripe structure 10 is significantly reduced. Thereby, asdescribed in the first embodiment, the threshold current is set toapproximately 20 μm in the short resonator having a length ofapproximately 50 μm.

[0106] However, when the insulator multi-layered thin films 11 areformed on the vertical faces, the film thicknesses of the insulator thinfilms become non-uniform when the layers are deposited, which leads toan increased reflection loss. Accordingly, the following structure ispreferable.

[0107]FIGS. 12A and 12B are views showing a structure of a modificationof the semiconductor light emitting device according to a thirdembodiment, in which FIG. 12A is a plan view showing a structure thereofand FIG. 12B is a view taken in the direction of arrows along line B-Bin FIG. 12A. In this modification, a reflection mirror is formed byphotonic crystal instead of the insulator multi-layered thin film.

[0108] As shown in FIGS. 12A and 12B, separation grooves 18 are formedin stripe in a plan view to extend from the p-type InP upper claddinglayer 5 to the n-type InP substrate 1. The photonic crystal structure 2and the upper electrode 6 are formed on the front surface of the p-typeInP upper cladding layer 5 within a region 17 surrounded by theseparation grooves 18. Cylindrical reflection concave portions 15 areformed in the shape of rectangular lattice in the p-type InP uppercladding layer 5 outside the region 17 so as to surround the region 17surrounded by the separation grooves 18. In both of the α and βdirections, a period of the reflection concave portions 15 (spacingbetween adjacent reflection concave portions 15) is shorter than aperiod of the concave portions 9. The reflection concave portions 15 areformed to extend from the upper cladding layer 5 to a part of the lowercladding layer 3. The region provided with the reflection concaveportions 15 becomes a reflection mirror region. The region 17 isseparated from the reflection mirror region by the separation grooves 18for the purpose of inhibiting leakage of a current from the region 17 tothe reflection mirror region.

[0109] When the reflection mirror is created by forming the insulatormulti-layered thin films on end faces of the stripe structure, only thelight in the direction coincident with the resonator direction isreflected. Therefore, such a reflection mirror is satisfactory for alaser. However, when the spontaneous emission light is controlled as inthe present invention, the spontaneous emission light in the directiondeviating slightly from the resonator direction cannot be reflectedsufficiently. On the other hand, in this modification, a refractiveindex of 98% is gained by forming the reflection concave portions 15 inapproximately four periods on one side in the reflection mirror region,while a refractive index of 95% is gained as shown in FIG. 12A, byforming the reflection concave portions 15 in approximately in twoperiods on one side. As a result, as described in the first embodiment,laser oscillation without a threshold current is achieved in a shortresonator having a length of approximately 20 μm.

Embodiment 4

[0110] In a fourth embodiment, there is shown a semiconductor lightemitting device capable of inhibiting a mode from becoming unstable dueto fluctuation of a phase of light on a reflection plane by forming aphotonic crystal structure over the entire stripe structure.

[0111]FIGS. 13A and 13B are views showing a structure of thesemiconductor light emitting device according to the fourth embodimentof the present invention, in which FIG. 13A is a plan view and FIG. 13Bis a view taken in the direction of arrows along line C-C in FIG. 13A.

[0112] As shown in FIGS. 13A and 13B, cylindrical concave portions 9 areformed in the shape of rectangular lattice over the entire p-type InPupper cladding layer 5 in stripe shape. Since the other structure of thesemiconductor light emitting device of this embodiment is identical tothat of the third embodiment, the same or corresponding parts areidentified by the same reference numerals and will not be furtherdescribed.

[0113] Subsequently, a method of fabricating the semiconductor lightemitting device of this embodiment structured as described above will bedescribed.

[0114] First of all, the photonic crystal structure 2 is created as inthe first embodiment. At this time, the cylindrical concave portions 9are formed over the entire upper surface of the stripe structure 10 bydry etching. Subsequently, the stripe-shaped upper electrode 6 is vapordeposited on the front surface of part of the photonic crystal structure2. Then, to form the resonator, the both end faces of the stripestructure 10 are formed to be vertical by dry etching, and then theinsulator multi-layered thin films 11 comprising alumina and amorphoussilicon are formed on the both end faces. Thereafter, the device isseparated along the dry-etched grooves, thereby obtaining thesemiconductor light emitting device of this embodiment as shown in FIGS.13A and 13B.

[0115] In the step of forming the upper electrode 6, to avoid entry ofthe electrode metal into an inside of the concave portion 9, instead ofvapor deposition of all the electrode metal, a thin Cr/Pt is depositedon the photonic crystal structure 2 to cause a contact resistance to bereduced, and thereafter, a metal foil comprising Pt/Au is bondedthereto, thereby forming the upper electrode 6. By the way, as shown inFIG. 6, when the concave portions 9 are formed to extend to a part ofthe lower cladding layer 3, an increase in leak current due to entry ofthe electrode material is significant. Accordingly, in this case, it ispreferable that after the thin SiO₂ film is deposited on the surface ofthe photonic crystal structure 2 and a surface of the flat portion ofthe SiO₂ film is removed by ICP dry etching using CHF₃, the upperelectrode 6 is vapor deposited.

[0116] In the semiconductor light emitting device so fabricated, uponapplication of a voltage between the upper electrode 6 and the lowerelectrode 7, stimulated emission occurs in the active layer 4 as in thefirst embodiment. Thereby, light is generated in the active layer 4.This light is amplified within the active layer 4 and diffracted withinthe photonic crystal structure 2 toward the direction perpendicular tothe n-type InP substrate 1. As a result, light is emitted from a regionof the photonic crystal structure 2 which does not overlap with theupper electrode 6, toward the direction perpendicular to the n-type InPsubstrate 1.

[0117] As should be appreciated, when the semiconductor light emittingdevice is modulated at a high speed by providing the photonic crystalover the entire upper surface of the stripe structure 10, standing wavesformed within the resonator become stable. This is because, when themodulation current is injected into the semiconductor light emittingdevice, the refractive index varies due to variation in carrier densitywithin the resonator, and thereby, the standing waves become unstable ina region without the photonic crystal. In this embodiment, by providingthe photonic crystal structure 2 over the entire upper surface of thestripe structure 10, perturbation of light is forcibly synchronized withthe period of the photonic crystal. Thereby, it has been found that atthe modulation speed of approximately 40 GHz, a stable light emittingmode is gained. However, when the resonator (active layer 4) has alength as large as 100 μm or more, light density within the resonatorhas a distribution in the resonator direction, so that an operationspeed is limited. It is therefore preferable that the length of theresonator is set to 100 μm or less.

[0118] When stability of the mode is important than reduction of thethreshold current, it is preferable that part of the cylindrical concaveportion 9 are flat concave portions 16, as shown in FIG. 14A. In FIG.14A, a plurality of flat concave portions 16 are formed on the regionwhere the upper electrode 6 is provided. Herein, an evaluation indexindicative of stability of the mode involves stability of a polarizationplane. In two-dimensional photonic crystal in which the cylindricalconcave portions 9 are arranged in the shape of rectangular lattice asshown in FIG. 13A, coupling of light in the B direction (directionperpendicular to the resonator direction) also occurs. For this reason,the polarization plane rotates due to variation in the lightdistribution in the B direction, so that the polarization plane becomesunstable. On the other hand, in the case of the photonic crystalstructure provided with the flat concave portions 16, perturbation ofthe spontaneous emission light in the B direction does not occur, andcoupling of only waves traveling in the a direction (resonatordirection) occurs. For this reason, the spontaneous emission light isnot sufficiently used and the threshold current increases. However,since light is subjected to perturbation with a uniform phase in thethickness direction of the stripe structure, the polarization plane doesnot rotate. As a result, the threshold becomes approximately 0.1 mA, butoutput light having a constant polarization plane is gained regardlessof intensity of output light.

[0119] On the other hand, when stability of the mode is important thanthe light intensity, two upper electrodes 6 are provided at both endportions of the stripe structure 10 except the center portion as shownin FIGS. 14B and 14C. Thereby, the center portion of the stripestructure 10 becomes a light output region. As can be seen from FIG.14E, in order to gain strong output light, it is desirable to take outlight from a vicinity of the reflection plane where the light intensitywithin the resonator is high. However, in the vicinity of the end face,hole burning occurs because light intensity increases rapidly as it iscloser to the end face, so that the polarization plane becomes unstable.Accordingly, as shown in FIGS. 14B and 14C, by providing the lightemitting region at the center of the resonator (active layer 4), theregion where the photonic crystal exists becomes a region having lowlight intensity, but the mode is stabilized because the hole burningdoes not occur.

[0120] Further, in order to improve stability of the polarization plane,as shown in FIG. 14D, flat concave portions 16 may be provided over theentire upper surface of the stripe structure 10. However, in such astructure, the polarization plane becomes constant, but the output lightis not sufficiently coupled with the light within the resonator and thespontaneous emission light is not efficiently used. As a result, the outlight is reduced.

[0121] While in this embodiment, the photonic crystal structure iscomprised of cylindrical or flat concave portions, the photonic crystalstructure may be comprised of cylindrical or flat convex portions. Inthis embodiment, the concave portions are used rather than the convexportions. This is because the concave portions reduces damage caused bydry etching, and when the electrode is formed on the photonic crystal,the crystal surface is continuous and flat, which makes it possible forthe electrodes to be easily formed. Especially in this embodiment, theconcave portions are preferably used. This is due to the fact that, ifthe photonic crystal structure is comprised of convex portions, thenelectrode metal is vapor deposited around the convex portions, andshortening might occur when etching is performed until it reaches thelower cladding layer or the active layer is selectively grown.

Embodiment 5

[0122] In a fifth embodiment, there is shown a semiconductor lightemitting device capable of further stabilizing the light emitting modeby incorporating a phase shift structure into the photonic crystalstructure.

[0123]FIGS. 15A to 15C are views showing a structure of thesemiconductor light emitting device according to the fifth embodiment ofthe present invention, in which FIG. 15A is a plan view showing thestructure, FIG. 15B is a cross-sectional view showing the structure of amodification, and FIG. 15C is a view showing change in light intensityin an internal portion of a stripe structure of the semiconductor lightemitting device.

[0124] As shown in FIG. 15A, the upper electrode 6 is formed on theupper surfaces of both end portions of the upper cladding layer 5, andis not formed on the center portion of the upper cladding layer 5.Therefore, in this embodiment, light emanates from the center portion ofthe stripe structure 10 (resonator).

[0125] Here, a spacing between adjacent concave portions 9 which arearranged in the resonator direction and located in a region other thanthe center portion of the resonator is represented by L. A spacingbetween adjacent concave portions 9 which are arranged in the resonatordirection and located at the center portion of the resonator isincreased to L+λ/4 n, where λ is a wavelength of light and n is anequivalent refractive index. Hereinafter, a structure with the spacingbetween concave portions increased by λ/4 n is called a λ/4 n shiftstructure.

[0126] Since the other structure of the semiconductor light emittingdevice of this embodiment is identical to that of the third embodiment,the same or corresponding parts are identified by the same referencenumerals and will not be further described.

[0127] By incorporating the λ/4 n shift structure in the resonatordirection, leftward wave and rightward wave are coupled, thereby causinglight intensity in the center portion of the resonator to be increasedas shown in FIG. 15C. As a result, since the light emanating regionbecomes symmetric, coupling to optical fibers is favorably facilitated.In addition, output light that is more intensive is gained by locatingthe light emanating region at the center portion of the resonator.

[0128]FIGS. 16A to 16B are views for explaining the light emitting stateof the light emitting device according to the fifth embodiment of thepresent invention, FIG. 16A is a view showing the relationship betweenwave number and energy of light in the photonic crystal structure in thea direction (resonator direction) in FIGS. 15A, and 16B is a viewshowing the relationship between wave number and energy of light in thephotonic crystal structure in the β direction (direction perpendicularto the resonator direction) in FIG. 15A.

[0129] As shown in FIG. 16A, the light energy obtained by coupling theleftward wave and the rightward wave to each other as described above isequivalent to the case where lattice defects occurs by incorporating theλ/4 n shift structure and a light energy level corresponding to defectlevel is formed within the photonic bandgap. When the λ/4 n shiftstructure is incorporated, energy corresponding to the lattice defect isderived from n_(eff)·ω₀/C=π/Λ. Therefore, this energy corresponds to anenergy at the center of the photonic band gap energy. The energy of thephotonic band gap is twice as high as Δω=2 kc/n_(eff) in the case wherethe concave portions 9 are arranged in the shape of rectangular latticeat equal intervals as shown in FIG. 14B in the fourth embodiment. Whenthe concave portions 9 are thus arranged in the shape of rectangularlattice at equal intervals, there is a possibility that light is emittedat the high-energy end and the low-energy end as shown in FIG. 10A, butby incorporating the λ/4 n shift structure, light is emitted at thecenter portion of the photonic bandgap where light is most easilyemitted. In the case of the λ/4 n shift structure, the high-energy sideand the low-energy side of defect are located within the photonicbandgap. Therefore, light is subjected to perturbation more greatly ascompared to the uniform lattice structure where the photonic bandgap isnot provided on one side in a light-emitting state shown in FIG. 7B.Therefore, the spontaneous emission light with a large Q value, a smallhalf width, and high intensity can be amplified in a specific mode.

[0130] In this embodiment, it has been revealed that, when thewavelength of the spontaneous emission light conforms to the defectlevel, it has been revealed that light emission due to strongperturbation is realized. By adjusting the period (spacing betweenadjacent concave portions) in the α and β directions, the spontaneousemission light level can be present within the photonic band gap in thelateral direction (direction perpendicular to the resonator direction).As a result, lateral light propagation becomes impossible and asingle-mode light emission in the resonator direction is observed.

[0131] The photonic crystal structure 2 may be comprised of thecylindrical concave portions 9 as described above or may be comprised ofthe flat concave portions 16 as shown in FIG. 15B. The use of the flatconcave portions increases a threshold current and intensity of outputlight, and improves stability of the polarization planes.

Embodiment 6

[0132] In a sixth embodiment, there is shown a semiconductor lightemitting device capable of further stabilizing the light emitting modeby providing stripe structures having the photonic crystal structures soas to cross each other.

[0133]FIGS. 17A and 17B are views for explaining a semiconductor lightemitting device according to a sixth embodiment of the presentinvention, in which FIG. 17A is a plan view showing a structure of thesemiconductor light emitting device, FIG. 17B is a view showing anoperation principle of the semiconductor light emitting device, and FIG.17C is a view showing behavior of an electric field.

[0134] In this embodiment, two stripe structures similar to that ofanother embodiments, are provided so as to cross each other. Morespecifically, as shown in FIG. 17A, as in another embodiments, a stripestructure 10A and a stripe structure 10B, which have the photoniccrystal structure, are arranged to extend respectively in the αdirection and the β direction (direction perpendicular to the αdirection) so as to cross each other. The upper electrode 6 is notprovided on a region B where the stripe structure 10A and the stripestructure 10B cross each other, while the upper electrode 6 is providedon a region A other than the region B.

[0135] Since the other structure of the semiconductor light emittingdevice of this embodiment is identical to that of the first embodiment,the same or corresponding parts are identified by the same referencenumerals and will not be further described.

[0136] Subsequently, a method of fabricating the semiconductor lightemitting device of this embodiment will be described.

[0137]FIGS. 19A to 19D are views showing a method of fabricating thesemiconductor light emitting device according to the sixth embodiment ofthe present invention, in which FIG. 19A is a cross-sectional viewshowing a structure of the semiconductor light emitting device, andFIGS. 19B to 19D are plan views showing a structure of the semiconductorlight emitting device.

[0138] As described with reference to FIG. 5A in the first embodiment,on the n-type InP substrate 1, the Si-doped n-type InP lower claddinglayer 3 (100 nm thick), the undoped active layer 4 (14 nm thick), andthe zn-doped p-type InP upper cladding layer 5 (50 nm thick) areepitaxially grown by the known crystal growth method such as the MOVPEprocess (FIG. 19A).

[0139] Subsequently, as in the first embodiment, using the SiO₂ film asthe etching mask, ICP dry etching is circularly performed using the Cl₂gas and the CH₄ gas, thereby forming a plurality of cylindrical concaveportions 9. In this case, as shown in FIG. 19B, the concave portions 9are arranged in the shape of rectangular lattice so as to form across-shape. The region with the plurality of concave portions 9arranged becomes the photonic crystal structure.

[0140] Subsequently, in order to form a stripe structure, etching isperformed until it reaches a part of the n-type InP substrate 1 from thep-type InP upper cladding layer 5 using an oxalic acid based etchant(FIG. 19C). Then, as shown in FIG. 19D, the Cr/Pt/Au electrode as theupper electrode 6 is vapor deposited on the front surface of the p-typeInP upper cladding layer 5 and the Au—Sn electrode as the lowerelectrode (not shown) is vapor deposited on the rear surface of then-type InP substrate 1. In this case, the upper electrode 6 is formed onthe surface of the p-type InP upper cladding layer 5 except the regionwhere the stripe structures 10A and 10B cross each other by lift-offprocess. Thereby, the stripe structure 10A and the stripe structure 10Bthat crosses the stripe structure 10A are created.

[0141] Through the above steps, the semiconductor light emitting deviceof this embodiment is fabricated.

[0142] Subsequently, an operation principle of the semiconductor lightemitting device of this embodiment will be described with reference toFIG. 17B. In FIG. 17B, an ordinate axis indicates energy and an abscissaaxis indicates a wave number. In the region A other than the region Bwhere the stripe structure 10A and the stripe structure 10B cross eachother, since the mode is locally present in the resonator direction,degeneration is relieved, and the high energy end splits into two bands.On the other hand, in the region B where the stripe structure 10A andthe stripe structure 10B cross each other, the α and β directions aresymmetric, and degeneration at one energy level occurs. Here, when lightin the region A is incident on the region B, light A1 on the low-energyside of the region A does not propagate because it is located within thebandgap of the region B, thereby resulting in a radiation mode. On theother hand, light A2 on the high energy side is amplified because it haspassed through the region B. By causing the light A2 in the region A toconform to the energy of the high-energy end of the region B in a regionB′ obtained by adjusting a diffraction efficiency κ and a period Λ ofthe region B, only the light A2 in the region A is amplified and takenout. In this case, as shown in FIG. 17C, behavior of the electric fieldsare such that the polarization plane rotates at a frequency of lightbecause variation in the electric field in the α direction and variationin the electric field in the β direction are perpendicular to eachother. In normal observation, the electric fields are perpendicular toeach other and the magnetic field and the electric field overlap witheach other. As a result, the direction of the electric field conforms tothe resonator direction, i.e., the direction of a period pointing vectorof the photonic crystal structure. In the case of a normal photoniccrystal device, the problem that the electric field deviates from theperiod pointing vector arises. In this embodiment, it has been foundthat there is an electric field perpendicular to the direction of thepointing vector.

[0143] The semiconductor light emitting device of this embodiment mayhave various structures described with reference to FIGS. 14A to 14D inthe third embodiment.

[0144]FIGS. 18A and 18B are views showing a structure of a modificationof the semiconductor light emitting device according to the sixthembodiment of the present invention, in which FIG. 18A is a plan viewshowing a structure of the modification and FIG. 18B is a plan viewshowing a structure of another modification. In the modification shownin FIG. 18A, a photonic crystal structure in the region A of the stripestructures 10A and 10B is comprised of flat concave portions 16, whilein the modification in FIG. 18B, a photonic crystal structure in theregion A of the stripe structure 10A is comprised of cylindrical concaveportions 9 and a photonic crystal structure in the region A of thestripe structure 10B is comprised of the flat concave portions 16. Inthis structure, emission of light from the region A (A1 in FIG. 17B) isinhibited.

[0145]FIGS. 20A to 20D are plan views showing a structure of anothermodification of the semiconductor light emitting device according to thesixth embodiment of the present invention. In the modification in FIG.20A, in the region B as a light output region, square-lattice-shapedconcave portions 19 are formed as seen in a plan view. In themodification in FIG. 20B, in the region B, concave portions 19 that aretubular with rectangular cross-section have a nested structure. It hasbeen found that in these structures, rotation of the pointing vector isinhibited. In particular, in order to inhibit the spontaneous emissionlight from leaking out from the stripe structure forming the resonatorstructure, cylindrical concave portions are formed outside the stripestructure so as to have a period adjusted to allow the wavelength oflight emission to be within the photonic bandgap. Thereby, lightemission efficiency can be increased.

[0146] In the modification shown in FIG. 20C, a photonic crystalstructure in which the wavelength of light emission is present withinthe photonic bandgap is formed around the resonator (stripe structure10A and stripe structure 10B). In this case, advantageously, ahigh-reflection film need not be provided on the end faces of theresonator. FIG. 21 shows a method of fabricating such a modification.

[0147]FIGS. 21A to 21D are views for explaining a method of fabricatingthe modification of the semiconductor light emitting device according tothe sixth embodiment, FIG. 21A is a cross-sectional view showing astructure of the modification, and FIGS. 21B to 21D are plan viewsshowing the structure of the modification.

[0148] As described with reference to FIG. 5A in the first embodiment,on the n-type InP substrate 1, the Si-doped n-type InP lower claddinglayer 3 (100 nm thick), the undoped active layer 4 (14 nm thick), andthe zn-doped p-type InP upper cladding layer 5 (50 nm thick) areepitaxially grown by the known crystal growth method such as the MOVPEprocess (FIG. 21A).

[0149] Subsequently, the plurality of flat concave portions 16 and thecylindrical concave portions 9 are arranged to form a cross-shapedregion. Specifically, as shown in FIG. 21B, the cylindrical concaveportions 9 are arranged in the shape of rectangular lattice at a centerportion of the cross-shaped region and the flat concave portions 16 arearranged at a predetermined spacing on the other region. And, around thecross-shaped region thus obtained, cylindrical reflection concaveportions 21 are formed in the shape of rectangular lattice so as tosurround the cross-shaped region.

[0150] As shown in FIG. 21C, separation grooves 22 are provided tosurround the cross-shaped region comprised of the flat concave portions16 and the cylindrical concave portions 9. As shown in FIG. 21D, anupper electrode is vapor deposited on a region other than the centerportion of the cross-shaped region.

[0151] Through the above steps, the modification of the semiconductorlight emitting device of the embodiment shown in FIG. 20C can befabricated.

[0152] The photonic crystal in the embodiments and the modificationsdescribed above is formed by arranging the concave portions in the shapeof square lattice or in the shape of rectangular lattice. Alternatively,as shown in FIG. 20D, the concave portions may be arranged in the shapeof triangular lattice. In this case, three stripe structures 10A, 10B,and 10C are provided so as to cross one another at an angle of 60degrees. Therefore, the region where the stripe structures 10A, 10B, and10C cross one another is hexagon-shaped. In this case, because there arethree resonator directions, triple degeneration occurs. For this reason,the photonic bandgap structure becomes complex, and design of thestructure becomes difficult. Nonetheless, advantageously, since thespontaneous emission light from most of the light-emitting region can beutilized, a high-output light emitting device is achieved.

[0153] Numerous modifications and alternative embodiments of theinvention will be apparent to those skilled in the art in the light ofthe foregoing description. Accordingly, the description is to beconstrued as illustrative only, and is provided for the purpose ofteaching those skilled in the art the best mode of carrying out theinvention. The details of the structure and/or function may be variedsubstantially without departing from the spirit of the invention.

What is claimed is:
 1. A semiconductor light emitting device comprising:a semiconductor substrate; a semiconductor layered structure provided onthe semiconductor substrate and comprised of a lower cladding layer, anactive layer having a resonator in a direction parallel to thesemiconductor substrate, and an upper cladding layer; an upper electrodeconnected to the upper cladding layer and extending in a stripe shape ina resonator direction; and a lower electrode connected to the lowercladding layer, wherein the semiconductor layered structure has aphotonic crystal structure in which a plurality of concave portions orconvex portions are arranged periodically in the resonator direction,the photonic crystal structure is configured such that at least part ofthe photonic crystal structure does not overlap with the upper electrodeand the photonic crystal structure and the upper electrode are arrangedin the resonator direction as seen in a plan view, and when apredetermined voltage is applied between the upper electrode and thelower electrode, light radiates from a region of the photonic crystalstructure which does not overlap with the upper electrode as seen in aplan view.
 2. The semiconductor light emitting device according to claim1, wherein the concave portions or the convex portions are formed in theupper cladding layer.
 3. The semiconductor light emitting deviceaccording to claim 1, wherein the concave portions or the convexportions are formed in the upper cladding layer, the active layer, andthe lower cladding layer.
 4. The semiconductor light emitting deviceaccording to claim 1, wherein the concave portions or the convexportions are cylindrical.
 5. The semiconductor light emitting deviceaccording to claim 1, wherein the concave portions or the convexportions are flat-plate shaped.
 6. The semiconductor light emittingdevice according to claim 1, wherein the resonator has a width of notless than 2 μm and not more than 10 μm.
 7. The semiconductor lightemitting device according to claim 1, wherein the resonator has a lengthof not less than 20 μm and not more than 50 μm.
 8. The semiconductorlight emitting device according to claim 1, wherein the resonatordirection is <110> direction or <−110> direction.
 9. The semiconductorlight emitting device according to claim 1, wherein the concave portionsor convex portions are arranged in the shape of rectangular lattice suchthat one arrangement direction of the concave portions or the convexportions corresponds with the resonator direction and anotherarrangement direction is perpendicular to the resonator direction. 10.The semiconductor light emitting device according to claim 9, wherein aspacing between adjacent concave portions or convex portions in the onearrangement direction is different from a spacing between adjacentconcave portions or convex portions in the another arrangementdirection.
 11. The semiconductor light emitting device according toclaim 10, wherein the spacing between adjacent concave portions orconvex portions in the one arrangement direction is larger than thespacing between adjacent concave portions or convex portions in theanother arrangement direction.
 12. The semiconductor light emittingdevice according to claim 1, wherein reflection films are provided onboth end faces of the semiconductor layered structure.
 13. Thesemiconductor light emitting device according to claim 1, wherein thesemiconductor layered structure is provided with a photonic crystalstructure on a periphery thereof, and the photonic crystal structure iscomprised of a plurality of concave portions or convex portions arrangedat a predetermined spacing.
 14. The semiconductor light emitting deviceaccording to claim 1, wherein the concave portions or the convexportions are provided over an entire upper surface of the semiconductorlayered structure.
 15. The semiconductor light emitting device accordingto claim 14, wherein the region of the photonic crystal structure thatdoes not overlap with the upper electrode as seen in a plan view islocated at a center portion of the semiconductor layered structure. 16.The semiconductor light emitting device according to claim 1, wherein aspacing between part of the concave portions or convex portions adjacentin the resonator direction is larger than a spacing between anotherconcave portions or convex portions by a wavelength/(actual refractiveindex×4).
 17. The semiconductor light emitting device according to claim1, comprising a plurality of semiconductor layered structures, whereinthe plurality of semiconductor layered structures are arranged to crossone another.
 18. A method of fabricating a semiconductor light emittingdevice comprising: a semiconductor substrate; a semiconductor layeredstructure provided on the semiconductor substrate and comprised of alower cladding layer, an active layer having a resonator in a directionparallel to the semiconductor substrate, and an upper cladding layer; anupper electrode connected to the upper cladding layer; and a lowerelectrode connected to the lower cladding layer, wherein light radiatesin a direction substantially perpendicular to the semiconductorsubstrate, the method comprising the steps of: epitaxially growing thesemiconductor layered structure on the semiconductor substrate; etchingthe semiconductor layered structure to form a photonic crystal structurecomprised of a plurality of concave portions arranged periodically in aresonator direction; and forming the upper electrode on the uppercladding layer so as to extend in stripe shape in the resonatordirection such that the upper electrode does not overlap with at leastpart of the photonic crystal structure and the upper electrode and thephotonic crystal structure are arranged in the resonator direction asseen in a plan view.
 19. A method of fabricating a semiconductor lightemitting device comprising: a semiconductor substrate; a semiconductorlayered structure provided on the semiconductor substrate and comprisedof a lower cladding layer, an active layer having a resonator in adirection parallel to the semiconductor substrate, and an upper claddinglayer; an upper electrode connected to the upper cladding layer; and alower electrode connected to the lower cladding layer, wherein lightradiates in a direction substantially perpendicular to the semiconductorsubstrate, the method comprising the steps of epitaxially growing thesemiconductor layered structure on the semiconductor substrate;selectively growing crystal on the upper cladding layer of thesemiconductor layered structure to form a photonic crystal structurecomprised of a plurality of concave portions arranged periodically inthe resonator direction; and forming the upper electrode on the uppercladding layer so as to extend in stripe shape in the resonatordirection such that the upper electrode does not overlap with at leastpart of the photonic crystal structure and the upper electrode and thephotonic crystal structure are arranged in the resonator direction asseen in a plan view.